This invention relates generally to managing differential focusing of projection displays including displays that may display computer information.
Projection displays may project an enlarged image on a display screen to be more easily viewable, for example, by a number of users. Projection display screens may also be mounted in housings with an image projected on one side of a screen, being viewed on the other side of the screen. In some cases, projection displays may be utilized as the displays for computer systems.
Projection displays project onto a display surface by angling the projection display with respect to a horizontal plane. Correct optical projection of the image assumes that the display surface is perpendicular to the axis of projection. Typically, the projection axis is at an angle to the projection surface. As a result, the image that appears on the projection surface no longer has a rectangular aspect ratio but instead is keystone shaped. This is a form of differential focus because the image is not focused everywhere. The keystone shaped image and its differential focus are different manifestations of the same problem. The keystone shaped image is a geometrical projection of a rectangle onto a plane at a angle.
Differential focus arises from the fact that the image plane distance varies because of the angle of projection. Due to the larger f numbers used by projectors, the angle of projection causes a shallow depth of focus.
If one attempts to correct the focus on the bottom portion of the keystone, the focus at the top of the keystone becomes worse and vice versa. There is no simple way to manually adjust the focus to correct the keystone effect.
Thus, there is a continuing need for ways of correct differential focus and keystone errors in projection displays.
In accordance with one aspect of the present invention, a method of projecting an image on a display surface includes projecting the image through an electrically variable optical element. The focal length of the optical element is adjusted to correct for differential focus.